The prior art teaches a number of spinal fixation systems which allow the spinal rod to be inserted after the pedicle screw is implanted with the angle between the rod and pedicle screw variable. An example of such systems is disclosed in European patent 330,881 of Sherman and European patent 441,729 of Vignaud. The disadvantage of these and similar systems is that the rod still must frequently be bent after coupling to the pedicle screw because of anatomical considerations. German patent 19,509,332 of Harms minimizes this disadvantage. Specifically, because relative movement of the screw head with respect to the rest of the screw is possible, the rod need only be bent in one plane after coupling with the pedicle screw.
The disadvantages of this known pedicle screw consist therein that the head part and the threaded part are not apt to be separated, the head part and the longitudinal rod are not apt to be fastened separately with respect to the threaded part, the reachable solidity is low because of the one-sided alignment of the longitudinal slots in the pressure element according to the DE-C 195 09 332 and it is difficult to remobilise the head part once it is fastened. The separability of the head part and the threaded part is a necessity to use different head parts according to the need and that the view is not impeded through the head part during the screwing in of the screw part into the bone. A separate fastening of the head part and the longitudinal rod and a high solidity between head part and threaded part enable the application of distraction and compression without a loss of angle between the pedicle screw and the longitudinal rod and the conservation of the anatomical bending of the stabilized vertebral segments. Remobilization is especially important in case of explantation or new adjustment of the fixation.
A further such connection device is disclosed in WO 97/02786 ERRICO which allows in a simple manner to connect a pedicle screw or more generally a bone anchoring element with a longitudinal rod and which permits a great freedom regarding the angle between the two elements. In case of this known connection device the disadvantage consists therein that the spring chuck is part of the connection element and therefore consists of the same material impairing the choice and pairing of the material with the bone screw to improve the solidity.